If you are curious what your eyeglass prescription means, begin with the headings “OS” and “OD.” These are Latin abbreviations for left eye (OS, oculus sinister) and right eye (OD, oculus dexter). The numbers listed under each refer to how much vision correction is needed. The further the number is from zero, the more vision correction you need. A plus (+) sign in front of the number indicates farsightedness, while a minus (-) sign indicates nearsightedness. As for the numbers themselves, they represent “diopters” (D), which are the units used to measure the correction (or focusing power) required by the lens. If you have astigmatism, there will be three numbers describing the type, degree, and orientation of the irregular curvature. Astigmatism is a type of refractive error (along with nearsightedness and farsightedness) that is caused by an eye that is not completely round.